Building a Resilient Financial Engine: How I Fixed a Data Duplication Bug with Idempotent SQL

Most developers hide their bugs. I prefer to document them. I recently built a full-stack financial dashboard to track budgets and spending. The goal was simple: connect a Python backend to a SQLite3 database and render the output dynamically on a web interface. But during testing, I hit a massive logic failure. My migration script was blindly inserting data every time it ran. Instead of my actual balance, my dashboard rendered a massive deficit of $-460.0 . Here is exactly how I refactored the database logic to enforce data integrity and deployed the final application to the cloud. The Tech Stack Backend: Python Database: SQLite3 Frontend: HTML5 / CSS3 Hosting: Hugging Face Spaces The Problem: Blind Data Insertion Initially, my Python script was executing simple INSERT statements without checking if the record already existed. Every time the app initialized, it duplicated the transactions. In a financial application, bad data is worse than a crashed app. A $-460.0 balance on a $500 bu